Currently paraffins, particularly aliphatic paraffins, are converted to olefins using thermal cracking technology. The thermal cracking of paraffins to olefins, particularly lower paraffins such as C2-4 paraffins typically ethane and propane to corresponding olefins is an energy intensive process. Typically the paraffins are passed through a furnace tube heated to at least 800° C., typically from about 850° C. to the upper working temperature of the alloy for the furnace tube, generally about 950° C. to 1000° C., for a period of time in the order of milliseconds to a few seconds. The paraffin molecule loses hydrogen and one or more unsaturated bonds are formed to produce an olefin. The current thermal cracking processes are not only cost intensive to build and operate but also energy intensive due to the substantial heat requirement for the endothermic cracking reactions which also results in greenhouse gas emissions. As a result, significant amounts of CO2 are produced from the operation of these cracking furnaces.
Dehydrogenation processes are widely used in modern refining and petrochemistry. Processes for the synthesis of butadiene, isoprene, and long-chain olefins are commercialized. However, the area of dehydrogenation of light alkanes remains to be underexplored and especially ethane dehydrogenation is far from the commercial scale. The most advanced are the processes of oxidative dehydrogenation based on the use of transition metal oxide catalysts and a robust oxidant, such as oxygen or air. The oxidative conversion makes the process of dehydrogenation thermodynamically advantageous and may decrease the reaction temperature as compared to non-oxidative processes (e.g. thermal cracking). The conversion of ethane, which is the second major component of natural gas, to ethylene requires development of new processes.
However, the technology of oxidative dehydrogenation of ethane has not been commercially practiced for a number of reasons including the potential for an explosive mixture of oxygen and paraffin at an elevated temperature. For satisfactory conversion of paraffins to olefins, the required oxygen in the feed mixture should be typically higher than the maximum allowable level before entering the explosion range. Another reason is the requirement of either front end oxygen separation (from air) or a back end nitrogen separation, which often brings the overall process economy into the negative territory. Therefore, solutions to address these issues are being sorted in various directions.
In the current prior art when a mixed feed of oxygen and hydrocarbon is used care must be taken so that the amount of oxygen in the mixture does not exceed about 25 Vol. % or the mixed feed will exceed an explosive limit. As far as applicants have been able to determine only a few patents related to the prior art in this field, it was suggested to use ethane oxidative dehydrogenation mixed oxide catalysts supported onto one side of the ceramic membrane as an important component of the ethane oxidative dehydrogenation process that allows one to segregate the hydrocarbon feed from the oxygen containing feed to minimize the potential for a mixture of oxygen and hydrocarbon to occur or if such mixture occurs to approach the explosive limit.
Several catalytic systems are known in the art for the oxidative dehydrogenation of ethane. U.S. Pat. No. 4,450,313, issued May 22, 1984 to Eastman et al., assigned to Phillips Petroleum Company discloses a catalyst of the composition LiO—TiO2, which is characterized by a low ethane conversion not exceeding 10%, in spite of a rather high selectivity to ethylene (92%). The major drawback of this catalyst is the high temperature of the process of oxidative dehydrogenation, which is close to or higher than 650° C.
The U.S. Pat. No. 6,624,116, issued Sep. 23, 2003 to Bharadwaj et al. and U.S. Pat. No. 6,566,573 issued May 20, 2003 to Bharadwaj et al., both assigned to Dow Global Technologies Inc., disclose Pt—Sn—Sb—Cu—Ag monolith systems that have been tested in an autothermal regime at T>750° C., the starting gas mixture contained hydrogen (H2:O2=2:1, GHSV=180 000 h−1). The catalyst composition is different from that of the present invention and the present invention does not contemplate the use of molecular hydrogen in the feed.
U.S. Pat. No. 4,524,236 issued Jun. 18, 1985 to McCain, assigned to Union Carbide Corporation and U.S. Pat. No. 4,899,003 issued Feb. 6, 1990 to Manyik et al., assigned to Union Carbide Chemicals and Plastics Company Inc. disclose mixed metal oxide catalysts of V—Mo—Nb—Sb. At 375-400° C. the ethane conversion reached 70% with the selectivity close to 71-73%. However, these parameters were achieved only at very low gas hourly space velocities less than 900 h−1 (i.e. 720 h−1).
Rather promising results were obtained for nickel-containing catalysts disclosed in U.S. Pat. No. 6,891,075 issued May 10, 2005 to Liu, assigned to Symyx Technologies Inc. At 325° C. the ethane conversion on the best catalyst (a Ni—Nb—Ta oxide catalyst) in this series was about 20% with a selectivity of 85%. The patent teaches a catalyst for the oxidative dehydrogenation of a paraffin (alkane) such as ethane. The gaseous feedstock comprises at least the alkane and oxygen, but may also include diluents (such as argon, nitrogen, etc.) or other components (such as water or carbon dioxide). The dehydrogenation catalyst comprises at least about 2 weight % of NiO and a broad range of other elements preferably Nb, Ta, and Co. While NiO is present in the catalyst it does not appear to be the source of the oxygen for the oxidative dehydrogenation of the alkane (ethane).
U.S. Pat. No. 6,521,808 issued Feb. 18, 2003 to Ozkan et al., assigned to the Ohio State University, teaches sol gel supported catalysts for the oxidative dehydrogenation of ethane to ethylene. The catalyst appears to be a mixed metal system such as Ni—Co—Mo, V—Nb—Mo possibly doped with small amounts of Li, Na, K, Rb and Cs on a mixed silica oxide/titanium oxide support. Again the catalyst does not provide the oxygen for the oxidative dehydrogenation rather gaseous oxygen is included in the feed.
U.S. Pat. No. 7,319,179 issued Jan. 15, 2008 to Lopez-Nieto et al., assigned to Consejo Superior de Investigaciones Cientificas and Universidad Politecnica de Valencia, discloses Mo—V—Te—Nb—O oxide catalysts that provided an ethane conversion of 50-70% and selectivity to ethylene up to 95% (at 38% conversion) at 360-400° C. The catalysts have the empirical formula MoTehViNbjAkOx, where A is a fifth modifying element. The catalyst is a calcined mixed oxide (at least of Mo, Te, V and Nb), optionally supported on: (i) silica, alumina and/or titania, preferably silica at 20-70 wt % of the total supported catalyst or (ii) silicon carbide. The supported catalyst is prepared by conventional methods of precipitation from solutions, drying the precipitate then calcining.
Similar catalysts have been also described in open publications of Lopez-Nieto and co-authors. Selective oxidation of short-chain alkanes over hydrothermally prepared MoVTeNbO catalysts is discussed by F. Ivars, P. Botella, A. Dejoz, J. M. Lopez-Nieto, P. Concepcion, and M. I. Vazquez, in Topics in Catalysis (2006), 38 (1-3), 59-67.
MoVTe—Nb oxide catalysts have been prepared by a hydrothermal method and tested in the selective oxidation of propane to acrylic acid and in the oxidative dehydrogenation of ethane to ethylene. The influence of the concentration of oxalate anions in the hydrothermal gel has been studied for two series of catalysts, Nb-free and Nb-containing, respectively. Results show that the development of an active and selective active orthorhombic phase (Te2M20O57, M=Mo, V, Nb) requires an oxalate/Mo molar ratio of 0.4-0.6 in the synthesis gel in both types of samples. The presence of Nb favors a higher catalytic activity in both ethane and propane oxidation and a better production of acrylic acid.
Mixed metal oxide supported catalyst compositions, catalyst manufacture and use in ethane oxidation are described in Patent WO 2005018804 A1, 3 Mar., 2005, assigned to BP Chemicals Limited, UK. A catalyst composition for the oxidation of ethane to ethylene and acetic acid comprises (i) a support and (ii) in combination with O, the elements Mo, V and Nb, optionally W and a component Z, which is metals of Group 14. Thus, Mo60.5V32Nb7.5Ox on silica was modified with 0.33 g-atom ratio Sn for ethane oxidation with good ethylene/acetic acid selectivity and product ratio 1:1.
A process for preparation of ethylene from gaseous feed comprising ethane and oxygen involving contacting the feed with a mixed oxide catalyst containing vanadium, molybdenum, tantalum and tellurium in a reactor to form an effluent of ethylene is disclosed in WO 2006130288 A1, 7 Dec., 2006, assigned to Celanese Int. Corp. The catalyst has a selectivity for ethylene of 50-80% thereby allowing oxidation of ethane to produce ethylene and acetic acid with high selectivity. The catalyst has the formula Mo1V0.3Ta0.1Te0.3Oz. The catalyst is optionally supported on a support selected from porous silicon dioxide, fused silica, kieselguhr, silica gel, porous and nonporous aluminum oxide, titanium dioxide, zirconium dioxide, thorium dioxide, lanthanum oxide, magnesium oxide, calcium oxide, barium oxide, tin oxide, cerium dioxide, zinc oxide, boron oxide, boron nitride, boron carbide, boron phosphate, zirconium phosphate, aluminum silicate, silicon nitride, silicon carbide, and glass, carbon, carbon-fiber, activated carbon, metal-oxide or metal networks and corresponding monoliths; or is encapsulated in a material (preferably silicon dioxide (SiO2), phosphorus pentoxide (P2O5), magnesium oxide (MgO), chromium trioxide (Cr2O3), titanium oxide (TiO2), zirconium oxide (ZrO2) or alumina (Al2O3).
The preparation of a supported catalyst usable for low temperature oxy-dehydrogenation of ethane to ethylene is disclosed in the U.S. Pat. No. 4,596,787 A, 24 Jun., 1986 assigned to UNION CARBIDE CORP. A supported catalyst for the low temperature gas phase oxydehydrogenation of ethane to ethylene is prepared by (a) preparing a precursor solution having soluble and insoluble portions of metal compounds; (b) separating the soluble portion; (c) impregnating a catalyst support with the soluble portion and (d) activating the impregnated support to obtain the catalyst. The calcined catalyst has the composition MoaVbNbcSbdXe. X is nothing or Li, Sc, Na, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, Y, Ta, Cr, Fe, Co, Ni, Ce, La, Zn, Cd, Hg, Al, Tl, Pb, As, Bi, Te, U, Mn and/or W; a is 0.5-0.9, b is 0.1-0.4, c is 0.001-0.2, d is 0.001-0.1, e is 0.001-0.1 when X is an element.
Other examples of the low temperature oxy-dehydrogenation of ethane to ethylene using a calcined oxide catalyst containing molybdenum, vanadium, niobium and antimony are described in the U.S. Pat. No. 4,524,236 A, 18 Jun., 1985 and 4,250,346 A, 10 Feb., 1981, both assigned to UNION CARBIDE CORP. The calcined catalyst contains MoaVbNbcSbdXe in the form of oxides. The catalyst is prepared from a solution of soluble compounds and/or complexes and/or compounds of each of the metals. The dried catalyst is calcined by heating at 220-550° C. in air or oxygen. The catalyst precursor solutions may be supported on to a support, e.g. silica, aluminium oxide, silicon carbide, zirconia, titania or mixtures of these. The selectivity to ethylene may be greater than 65% for a 50% conversion of ethane.
The application of dense ionic oxygen conducting membrane reactors (IOCMR), where both separation and reaction are integrated in a same unit is described in the papers by J.-A. Dalmon et al./Applied Catalysis A: General 325 (2007) 198-204; F. Akin, Y. Lin, J. Membr. Sci. 209 (2002); H. Wang, Y. Cong, W. Yang, Catal. Lett. 84 (2002) 101; M. Rebeilleau-Dassonneville et al. Catalysis Today 104 (2005) 131-137. However, the mechanism of the reaction involves the dissociation of oxygen, migration of O2− species and backward electron transfer through the dense ionic membrane and further oxidation of ethane producing ethylene. Although the ethane conversion on V/MgO/membrane or Pd/membrane catalysts reached about 100% at 1150K, the selectivity drops from 95% at 950K, when the reaction started to 60% at 1150K, and the total feed to the reaction side of the membrane was as low as 37 ml/min at P(ethane)/P(O2) ˜0.255, so the overall oxidation process was very slow and had a negligible productivity. Also the reaction temperatures were extremely high (950-1150 K).
The authors (D. Farrusseng) describe the preparation of such ionic conducting membranes for the oxidative dehydrogenation of alkanes in WO2006/024785A1, Mar. 9, 2006.
A similar ionic membrane reactor for the catalytic oxidation of alkanes was disclosed in U.S. Pat. No. 6,730,808 A1, May 4, 2004, assigned to BASF. However, the only example given in the patent describes the oxidation of butane into maleic anhydride.
A model was developed for the electrochemical oxidative dehydrogenation of ethane to ethylene in a solid electrolyte membrane reactor (L. Chalakov, Chemical Engineering Journal 145 (2009) 385-392, but the process occurs at T˜600° C., and the selectivity is lower than 50%.
Fluidized bed membrane reactor (FLBMR) was disclosed for the catalytic oxidative dehydrogenation of ethane using a γ-alumina supported vanadium oxide catalyst (D. Ahchieva, Applied Catalysis A: General 296 (2005) 176-185). The maximum ethylene yield observed in this type of the reactor was 37% and a favorable operation range with respect to the oxygen-hydrocarbon ratio was observed, which indicates a lower sensitivity against oscillations and disturbances in the reactant feed, corresponding to a higher safety of operation. However, the W/F (mass of catalyst per unit volumetric gas flow rate) ratio was too high—between 150 and 230 kg/m3, the selectivity to ethylene was below 70% and significant amounts of CO and CO2 were found in the products. Also, the reaction temperature was too high for the commercial realization (˜600° C.).
A membrane catalytic reactor has been tested for the oxidative dehydrogenation of ethane (J. Coronas, Ind. Eng. Chem. Res. 1995, 34, 4229-4234). This reactor consists of a fixed bed of Li/MgO catalyst encompassed by a porous ceramic membrane. Oxygen was permeated through the membrane while ethane was fed axially. Two different configurations of the membrane reactor were tested: a homogeneous wall membrane reactor and a mixed system which was equivalent to a membrane reactor followed by a conventional fixed bed reactor. Using this system, high conversions of ethane were obtained, while maintaining a good selectivity. This gave yields to ethylene and higher hydrocarbons of up to 57%. In addition, the membrane reactor allowed a safe and stable operation, even when a relatively high proportion of oxygen was used in the overall feed. The major drawback of this system was a high temperature of the process (650-750° C.) and substantial coke formation suppressing the stability and life time of the catalyst.
Thus, none of the above art teaches or suggests the use of a continuous process of oxidative dehydrogenation of ethane using a mixed oxide catalyst containing vanadium, molybdenum, niobium, tellurium, and antimony supported onto one surface of the porous ceramic membrane in which oxygen and gaseous paraffin feeds are supplied separately to opposite sides of the membrane.